This invention generally relates to electronic systems and in particular it relates to voltage regulator circuits.
Linear regulators that use NPN output drivers must be able to drive the base of the NPN transistor, which could mean potentially high current values. They must also be able to provide the displacement current needed to drive the load capacitance, as well as the parasitic capacitance, during transients. The typical prior art circuit used to drive an output NPN transistor is either an NPN emitter follower or NMOS source follower gain stage as shown in FIG. 1. The prior art circuit of FIG. 1 includes NMOS transistor 20; NPN output driver 22; amplifier 24; resistors 26 and 27; load capacitance CL; current IL; bias current Ibias; reference voltage Vref; supply voltage Vs; output voltage Vout; and ground gnd. This solution requires that the supply voltage Vs be at least a gate-to-source voltage (Vgs) (or a base-to-emitter voltage (Vbe) for an NPN common-emitter circuit) above the voltage at the base of the output NPN transistor 22 (which is a Vbe above the regulated output voltage Vout). This voltage could be quite large, especially if the required current is in the milliamp range.
In another potential solution to the problem, an amplifier could be designed to drive the base current and maintain the low impedance at the output; however, that solution would be more complex, requiring more area and potentially more quiescent current, and a high output current output stage.
A voltage regulator circuit that provides the current necessary to drive an output driver during transients and maintain low output impedance, while having a much better dropout voltage than a single source follower gain stage includes: an output driver; a source follower for controlling the output driver; a localized feedback gain loop coupled to the source follower; and an amplifier for controlling the source follower.